Align one face to another face of object?
In Object Mode select the plane that you want to align (the one that you want to change).
Add the second plane to the selection so that it is the active object.
In the 3D View Properties panel right click on one of the Rotation values and select 'Copy to Selected from' the pop-up menu and they will have matching rotation values.


I think that there are several ways to do this depending on the situation. This method may not work for all scenarios but it will work for two planar objects.